The function of synovial fluid is to lubricate the joints and to act as a medium for nutrients to maintain the cartilage. The cells that maintain the cartilage have no blood, nerves or lymphatic ducts connected to them, so the synovial fluid is the only way they are supplied.
The synovial fluid is primarily filtered from blood fluids, with a large amount of hyaluronic acid added. This acid gives the synovial fluid a rather thick viscous consistency. Without it, the fluid has about the same consistency as water. It also contains about as much glucose as the blood. Indeed, it matches the chemical makeup of serum in many areas and contains mostly the same proteins in lower amounts, but it lacks most blood cells. It also lacks any coagulation proteins. It does contain some lymphocytes, or white blood cells, which fight pathogens.
The synovial fluid lubricates joints by introducing a slight separation between the joint's cartilage surfaces. The chemicals in synovial fluid cause it to stick to and coat synovial surfaces. With industrial lubricants such as motor oil, the moving parts always go in the same direction and so pull a wedge of fluid between them. Joints in animals move in many directions and so cannot use this mechanism.